The long term objective of the proposed work is to evaluate the future potential of making targeted gene modifications by homologous recombination for the somatic therapy of beta thalassemia and sickle cell anemia. Four specific aims have been selected which in combination should assist in the development of a gene targeting mode of therapy. Each specific aim also explores some aspect of gene manipulation by homologous recombination that either has intrinsic interest outside the field of gene therapy, or should yield benefits not only for hemoglobinopathies but also for other genetic diseases. (i) DNA constructs will be developed with the specific aim of targeting beta globin genes efficiently while still maintaining their normal expression. (ii) Gene targeting in embryonic stem cells will be used to make a mouse strain having the human delta and betas globin genes in place of the mouse adult beta globin genes with the specific aim of providing animals for in vivo test of constructs designed to correct human globin genes. (iii) with the specific aim of developing conditions for gene targeting in bone marrow stem cells, a mutant hypoxanthine phosphoribosyl transferase (HPRT) gene will be corrected in these stem cells. (iv) A procedure will be tested to give targeted cells a controlled proliferative advantage over unaltered cells, with the specific aim of ensuring that cells returned to a recipient after gene targeting can effectively compete in vivo with uncorrected resident cells.